Recently, as office automation has developed, copying machines or printers using various recording systems such as an electrophotographic system, an ink jet system or a heat transfer recording system have come into use depending on the intended application. A coloring material is used to form an image. Usually, a composition containing a coloring material is melted or a coloring material is evaporated or sublimated to form an image on a recording medium such as a paper or a film sheet by adhesion, adsorption or dyeing.
In particular, a heat-sensitive transfer recording system is advantageous in that operation and maintenance are easy, and the apparatus can be down-sized or reduced in cost. This heat-sensitive transfer recording system has conventionally been known to include the following two types of systems. One type is a heat-fusion type transfer system where a transfer sheet comprising a support having thereon a heat-fusible ink layer is imagewise heated by laser beams or a thermal head to melt-transfer the fusible ink onto an image-receiving sheet for heat-sensitive transfer recording. The other type is a sublimation type dye transfer system where an ink sheet for heat-sensitive transfer recording comprising a support having thereon an ink layer containing a heat-diffusible dye (sublimation dye) is used, and the heat-diffusible dye is diffusion-transferred into an image-receiving sheet for heat-sensitive transfer recording.
The sublimation type dye transfer system is advantageous in that the gradation of an image can be controlled by varying the transfer amount of a dye depending on the change in thermal energy of the thermal head. Therefore, a color image having a continuously variable color density can be obtained by the superimposed recording of cyan, magenta and yellow images. However, this system has the following disadvantages:
(1) Because the image gradation is mainly reproduced by varying the density gradation, the system may satisfy a limited demand in public use where gradation similar to that of a photograph is desired. However, this technique is not suitable for color proofing, for example, in the printing field where the gradation is reproduced by area gradation alone; PA1 (2) Because the image formation is achieved by sublimating a dye, the finished image does not have satisfactory edge sharpness, and the solid density of a thin line is lower than that of a thick line. This gives rise to poor character image quality; PA1 (3) Because the resulting image has low durability, expanded use into product applications requiring good heat durability or light resistance is restricted; PA1 (4) Because its heat-sensitive recording sensitivity is low compared to that of a heat-fusion type transfer method, the sublimation type dye transfer system is not suitable for future high-speed recording applications using a high-resolution thermal head; and PA1 (5) A sublimation type dye transfer material is expensive compared to a heat-fusion type transfer material. PA1 (1) Because the gradation is reproduced by binary recording without density gradation, the multiple gradation property is inferior; PA1 (2) Because a crystalline wax having a low melting point is usually used as a binder of the ink layer, the resolution is reduced due to bleeding of the wax upon heat printing and the transfer image has low intensity; and PA1 (3) If a crystalline wax is used, a transparent image is difficult to obtain due to light scattering of the crystal phase.
On the other hand, a heat-fusion type transfer system is advantageous in that its heat-sensitive sensitivity is high compared to a sublimation type dye transfer system, the light resistance of the resulting image is excellent and the material is inexpensive. However, this system has the following disadvantages.
Under these circumstances, the present inventors previously proposed a heat adhesion thin film release system as a new type of heat-sensitive recording material. Such a system is capable of providing a pigment color image having a multiple gradation property only using the area gradation of binary recording JP-A-7-117359 (the term "JP-A" as used herein means an unexamined published Japanese patent application). According to this system, a high-quality color image or monochrome image having multiple gradation can be achieved by a pigment transfer method using area gradation alone. This considerably alleviates the problems encountered in a conventional heat-sensitive transfer recording system, and thereby allows for public use as well as expanded use into the printing field for color proofing or as a block copy original. In addition, due to the light resistance of the pigment, this system can be advantageously used in the card field, the outdoor display field or the meter display field.
On the other hand, various transfer materials have been proposed for use in the above-described heat-sensitive transfer recording materials. When plain paper is used in an image-receiving medium of a sublimation type dye transfer system, the dyeing is particularly difficult. As a result, not only is the density of the recorded image low, but there is also a conspicuous discoloration phenomenon caused by aging. Accordingly, a plain paper having provided thereon an image-receiving layer comprising a thermoplastic resin as a main component has been proposed as an image-receiving paper, and various investigations are being conducted to improve recording sensitivity, resolution, sharpness, color density and the like. A plain paper can be used in principle as a transfer material in a heat-fusion type transfer system. However, the use of plain paper is accompanied by uneven transfer, dot falling or the like defect due to smoothness of the transfer surface or ink acceptability. Accordingly, various image-receiving materials are being investigated so as to improve surface smoothness, ink transfer acceptability, fixability, gradation property, sharpness and the like. The support for these image-receiving media uses a material such as a plain paper, a synthetic paper, a synthetic resin film or a white base filled with a white pigment. In this way, the material on which a transfer image is formed is restricted to an exclusive paper or resin sheet having provided thereon an image-receiving layer. Therefore, the quality is unsatisfactory if it is used for proofing which requires printing matter approximation.
A method for transferring a recording image to arbitrary desired supports is also known. JP-A-52-27642 describes a method where a transfer layer is attached by heating to an intermediate receptor, and then the transfer layer thus attached is transferred to a material such as paper. However, this publication does not mention any improvement in image quality.
JP-A-63-82786 describes a technique for selecting a resin or wax as well as a butyral resin contained in the heat transfer layer as a component of the coating film other than a butyral resin. Examples of the resin include a phenol resin, an epoxy resin, a urethane resin, a rosin or polyamide resin and a sulfonamide resin. However, when using the above-described resin, particularly a rosin or polyamide resin, the shadow part vanishes. Furthermore, when a polyamide resin is used, the reproduced gradation property is highly sensitive to changes in temperature and humidity. Another problem is that when the resin is added in a large amount, the surface of the image-receiving layer becomes tacky. This in turn causes an increase in dust flaws or unevenness.
Thus, although various image-receiving sheet materials have hitherto been proposed as a transfer material of a heat-sensitive transfer recording material, none of these has achieved an image having excellent recording sensitivity, dot quality and gradation reproducibility in a heat transfer method using area gradation. In particular, if the heat recording sensitivity is elevated, the heat adhesion temperature of the image-receiving layer is lowered and tackiness tends to increase. This gives rise to a sticking phenomenon due to tackiness or reduced resistance to adhesion of the image upon storage after recording. As a result, the printed matter approximation such as texture reproduction of the printing permanent paper is unsatisfactory, especially for obtaining a high quality color image having multiple gradation such as a color proof for color correction.
Also, investigations have been made on a heat transfer ink layer. For example, JP-A-5-254256 describes the formulation of a toner layer comprising a polyamide resin (nylon) binder and a pigment. However, the high cohesion of the nylon gives rise to poor dot sharpness and gradation reproducibility, or inferior resistance to changes in temperature and humidity.
As discussed above, there has not yet been known an image-receiving material for heat-sensitive transfer recording that is capable of forming a high-quality multiple gradation color image having good recording sensitivity, dot quality (halftone dot quality) and gradation reproducibility (halftone reproducibility), which image receiving material does not exhibit tackiness, sticking and dust flaws, and wherein the change in recording sensitivity to changes in humidity is reduced and the printed matter approximation is excellent.